FIG. 2 is a circuit diagram illustrating a part of a general configuration of a step-down DC-DC converter. The DC-DC converter shown in FIG. 2 is a step-down DC-DC converter, which uses an N-channel Metal Oxide Semiconductor Field Effect Transistor (MOSFET) at a high-side, and a N-channel MOSFET for synchronous rectification is connected at a low-side to reduce a loss of a flywheel diode D12 and to increase the power conversion efficiency. An input voltage Vin is applied to a drain terminal of the high-side MOSFET Q1, and the drain terminal of the low-side MOSFET Q2 is connected to a source terminal of the high-side MOSFET Q1. One end of a coil L11 is connected to a connection point where the high-side MOSFET Q1 and the low-side MOSFET Q2 are connected, and a smoothing capacitor C12 and a load RL are connected between the other end of the coil L11 and the GND. The coil L11 and the smoothing capacitor C12 configure a direct current smoothing circuit at an output part of the DC-DC converter. The drive circuit of the high-side MOSFET Q1 comprises a bootstrap capacitor C11, which supplies an approximately constant voltage to a buffer circuit BF1 based on the electric potential of the source terminal of the high-side MOSFET Q1 (the electric potential of the terminal SW shown in FIG. 2), in order to provide a voltage higher than the input voltage Vin to the buffer circuit BF1 and the gate terminal of the high-side MOSFET Q1.
In the DC-DC converter, the high-side MOSFET Q1 and the low-side MOSFET Q2 are controlled to be ON/OFF, in response to inputting a high-side drive signal into the gate terminal of the high-side MOSFET Q1 through the buffer circuit BF1 and inputting a low-side drive signal into the gate terminal of the low-side MOSFET Q2 through a buffer circuit BF2.
In the above-described DC-DC converter, due to use the N-channel MOSFET as a switching element at the high-side of the power supply device, it is necessary to raise the voltage of the gate electrode of the MOSFET to be higher than the input voltage Vin. A bootstrap circuit, which uses the bootstrap capacitor C11, is used for this purpose. However, when the terminal to which the input voltage Vin is applied and the terminal to which the bootstrap voltage is applied (the terminal BS shown in FIG. 2) are shorted by adhesion of dust or the like, since the input voltage Vin is applied between the terminal BS and the terminal SW, each of a withstand voltage of the buffer circuit BF1 between the terminal BS and the terminal SW, a withstand voltage between the gate and the source of the high-side MOSFET Q1 and a withstand voltage between the gate and the drain of the high-side MOSFET Q1 are exceeded, and thus the buffer circuit and MOSFET Q1 may be broken down and shorted.
Such short between the drain and the source of the high-side MOSFET Q1 due to the breakdown of withstand may be caused when the low-side MOSFET Q2 is turned on, in which the voltage of the terminal SW becomes Low. At this time, because a through current flows through the high-side MOSFET Q1 and the low-side MOSFET Q2 from the Vin to the GND, the Area of Safe Operation (ASO) breakdown of the low-side MOSFET Q2 is also caused by the overcurrent. Further, when the low-side MOSFET Q2 is broken, excessive current flows between the Vin and the GND, and the product may be broken due to intense damages.
In view of this, a protection circuit is proposed. When the voltage of the bootstrap circuit increases due to some abnormality as above and thus the switching elements cannot be normally turned on and off, the protection circuit interrupts the power supply to the switching elements, and thus safety is improved by suppressing damages of the switching elements (for example, refer to JP-A-2006-280014).
FIG. 1 is the circuit diagram of an example of power supply devices including a protection circuit and a bootstrap circuit. In FIG. 1, a bootstrap circuit 106 includes capacitors C5 and C6, and a diode D3. Further, the protection circuit includes a diode D5 as a voltage detecting part, a transistor Tr1 and a fuse F2.
Behaviors of the bootstrap circuit 106 will be described below. The GND of the bootstrap circuit 106 is a floating GND, which is connected to the source terminal of a switching element S2. Thus, the terminal Vb of the bootstrap circuit 106 is always kept higher than the source terminal voltage of the switching element S2 as the terminal voltage Vboot, namely the charging voltage of the capacitor C6, regardless of ON/OFF of the switching element S2. This is because the capacitor C5 is charged by the capacitor C6 through the diode D3 when the switching element S2 is turned on.
When the bootstrap circuit 106 works normally, the bootstrap circuit 106 control the switching element S2 to turn ON/OFF by repeating the charging and discharging of the capacitors C5 and C6. On the other hand, when the diode D3 in the bootstrap circuit 106 is shorted due to some abnormality and the voltage of the terminal Vboot of the control IC increases, the diode D5 is turned on and thus the transistor Tr1 is turned on. Therefore, the input and the GND are shorted, and the fuse F2 is meltdown. The power supply to the switching element S2 is interrupted by such behaviors, and operations of the switching element S2 is stopped. Therefore, damages of the switching element S2 is to be suppressed and safety is to be improved.